VITAMIN D

any of a group of fat-soluble alcohols important in calcium metabolism in animals. They are formed by ultraviolet irradiation (sunlight) of sterols present in the skin. The most important of these sterols are 7-dehydrocholesterol, formed by metabolic processes in animals, and ergosterol (q.v.), present in vegetable oils. The action of sunlight converts these two compounds, respectively, to cholecalciferol, or vitamin D3, and calciferol, also called ergocalciferol, or vitamin D2. Vitamins D2 and D3 are equivalent in human metabolism, but in birds vitamin D2 is much less effective than D3, which therefore is preferred in the formulation of poultry-feed supplements. After the vitamins are formed from the sterols, further chemical reaction, which occurs in the liver, changes them into compounds that participate in absorption of calcium into the bloodstream and formation of bone. It is not usually essential that vitamin D be taken in food, but winter sunshine in northern regions and sunlight that has passed through certain kinds of glass or through clouds or the contaminated air of cities may lack sufficient amounts of ultraviolet rays to bring about adequate production of the vitamin. Under these conditions, supplementary vitamin D must be ingested. In the United States and Canada it is customarily added to milk. Vitamin D is one of the most active vitamins in terms of the amount required per day. About 10 micrograms (400 International Units) is considered adequate for a growing child. Lack of vitamin D causes rickets (q.v.). A prolonged high intake of the vitamin can cause a toxic reaction (see vitamin D excess).